As for the single gene that differentiates dullards from the near geniuses, well even that could work through what Ned Block has called indirect effects. Assume that the gene in question is a red hair gene in an anti red world:
"Consider a culture in which red haired children are beaten over the head regularly, but all other children are treated well. This effect will increase the heritability of IQ because red haired identical twins will tend to resemble one oanother in IA (because they will both have low IQs) no matter what the social class of the family in which they are raised. The effect of a red hair gene on red hair is 'direct' genetic effect because the gene affects teh color via na internal biochemical process. By contrast, a gene affects a characteristic *indirectly* by producing a direct effect which interacts with the environment so as to affect the characteristic. In the hypothetical example, the red hair genes affect IQ indirectly. In the case of IQ, no one has any idea how to separate out direct from indirect genetic effects because no one has much an idea how genes and environment affect IQ. For that reason, we don't know whether or to what extent the roughly 60% heritability of IQ found in whites populations is *indirect heritability*."
>From *Cognition* 56 (1995) 99-128
This is one of the many problems in any claim of genetic determination. I
have not yet read Sahotra Sarkar's Genetics and Reductionism; Stephen
Rose's *Lifelines* raises many other important problems as does Garland
Allen in the Endeavour piece I already cited here.
rb